ORDER TESTACEA. 103 
duosa, Chemn., in its fantastic figure and unequally oblique 
valves. 
PECTUNCULUS, Lam., 
The hinge ferming a curved line, and the shell lenticular; its 
valves always close completely, and their summits are ap- 
proximated. The animal AxIMEA, Pol?, is furnished with a 
large compressed foot with a double inferior margin, which 
enables it to crawl. They live in ooze. Some species are 
found upon the coast of France. 
Nucu.La, Lam. 
The nuculz are arc in which the teeth are arranged in a 
broken line. Their form is elongated and narrowed near the 
posterior extremity. Their animal is unknown, but is pro- 
bably not far removed from those of the preceding shells, 
(arca pellucida,) Chemn. 
This has long been the place assigned to the 
TRIGONIA, brug., 
So remarkable for the hinge, which is furnished with two 
plates like a chevron, crenulated on both faces, each of which 
penetrates into two cavities, or rather between four plates of 
the opposite side, similarly crenulated on the internal surface. 
The internal impressions on the shell had already warranted 
the supposition that the animal was not provided with long 
tubes. Messrs. Quoy and Gaymard have lately discovered 
living specimens of this genus, and, in fact, its mantle, like 
that of the arce, is open, and without any separate orifice 
even for the anus. The foot is large, its anterior portion 
trenchant, and like a hook. 
The living trigoniz resemble the cardiz in the form of their 
shell and the ribs which furrow it: its anterior is composed 
of mother-of-pearl. (Trigonie nacrée, Lam.) 
